The present invention relates to a process for the decomposition of complexes formed from orthobenzoyl-benzoic acid (OBB acid), hydrogen fluoride (HF) and boron trifluoride (BF.sub.3), of the general formula: ##STR1## in which n and m are between 1 and 6.
Such complexes can be obtained according to the European Patent Application No. 0,055,951.
In order to be able to isolate the OBB acid contained in such complexes, with a view essentially to cyclize it into anthraquinone, these must be subjected, for example, to treatment with boiling water or sodium hydroxide. A treatment of this nature has the major disadvantage in that it causes destruction of the HF and of the BF.sub.3 and consequently means that recovery of the HF/BF.sub.3 catalyst used in the synthesis of anthraquinone described in the above-mentioned European Patent Application is not economically viable.
Prolonged heating of the complex at 150.degree. C. to 200.degree. C. makes decomposition possible, but at the same time causes almost total degradation of the OBB acid itself-moreover, the water water formed in the course of this degradation combines with the HF and the BF.sub.3 and thus renders the catalyst unsuitable for direct recycling.
Prolonged heating of the complex under vacuum at a lower temperature, for example 50.degree. C. to 100.degree. C., makes it possible to limit the decomposition of the OBB acid, but allows some of the catalyst to remain in this OBB acid, which renders the process uneconomical.
Heating of the complex at a low temperature in an inert solvent still only leads to incomplete recovery of the catalyst and still causes significant degradation of the OBB acid.
French Patent Application No. 82/14,920 describes a process for the decomposition of the complex in which the complex is subjected to the action of an inert solvent at a temperature of at least 20.degree. C. in a distillation column functioning with vigorous reflux of the solvent. Such a process allows virtually quantitative recovery of the HF and the BF.sub.3, but has the disadvantage that the OBB acid can be isolated only after it has been separated from the solvent which acts on the complex.